Ultrafast Electron-Transfer and -Trapping Dynamics in the Inter-Band-Gap States of ZrO2 Nanoparticles Sensitized by Baicalein

Abstract

Charge-transfer dynamics of baicalein (BIC) adsorbed on TiO2 and ZrO2 nanoparticles have been studied by steady-state and time-resolved emission spectroscopy and femtosecond transient absorption spectroscopy. Steady-state absorption and emission studies indicate that the BIC molecule forms a charge-transfer (CT) complex with both TiO2 and ZrO2 nanoparticles through its pyrogallol moiety and quinone moiety, respectively. On exciting the CT complex a new charge-transfer emission band was detected only in the BIC–ZrO2 system not from the BIC–TiO2 system. On photoexcitation of BIC–TiO2 and BIC–ZrO2 systems by an ultrafast laser pulse, electron injection into the nanoparticles has been confirmed for both systems by direct detection of the electron in the nanoparticle and cation radical of BIC (BIC•+) in the transient spectra. Ultrafast transient decay kinetics of the BIC•+ cation radical suggests that back electron transfer (ET) dynamics is multiexponential on both nanoparticle surfaces. Due to the fact that the first excited state (S1) state lies below the conduction band edge of ZrO2, our measurements suggest that in the case of ZrO2 electrons are directly injected into the surface states and getting trapped in different surface states with a time constant of 900 fs before slow recombination with BIC cation radical. From the above investigation we suggest that electron injection into the surface states of nanoparticles is only feasible or facilitated when the adsorbate forms a strong CT complex with the nanoparticles.